Method for continuously filtering raw brine for use in chlor-alkali electrolysis

ABSTRACT

Disclosed is a method for continuously filtering raw brine (A) for use in chlor-alkali electrolysis by means of an all-automatic, backwashing pressure filter ( 5 ). In a first step, raw brine (A) is separated into a partial flux with a low solids content as clear brine (B) and a partial flux (C) with a high solids content via a decanter ( 1 ). In a second step, one portion of the partial flux (C) of the clear brine (B), which has a high solids content, is added to the pressure filter ( 5 ) as an auxiliary filtering means.

The invention relates to a method for continuously filtering raw brinefor use in chlor-alkali electrolysis by means of a backwashing pressurefilter.

During conversion to membrane cell technology in chlor-alkalielectrolysis the demands on the brine qualities have increasedconsiderably. In order to protect downstream plant components andelectrolysis cells, inter alia solid, suspended impurities have to beremoved down to a very low level. This solid/liquid separating task isfrequently carried out nowadays by multi-stage and correspondinglyexpensive separating methods, high demands being placed on thereliability primarily in the last separating stage.

Owing to the high investment and operating costs for such plant,alternative concepts which allow method simplification are becomingincreasingly important. In addition, the fact plays a part that manyinstallations are conversion operations replacing available obsoleteelectrolyses by the more efficient and environmentally friendly membraneplant. In these cases an extensive use of available separating apparatusis aimed for. The local conditions, such as infrastructure, spacelimitations; etc. also have to be considered here. The shutdown ordisassembly of available separating apparatus (primarily large-volumethickeners) is often also not desirable at all or is-connected with veryhigh expenditure.

Raw brine contains insoluble suspended solids originating fromdissolving basins or precipitation reactions. Typical components of rawbrine are barium sulphate, calcium sulphate, calcium carbonate, metalhydroxides and gangue. The concentration and composition of theimpurities depends primarily on the salt source and can vary sharplydepending on location. Typical concentrations for brines made of rocksalt of European origin are in the region of 300 to 1,500 ppm suspendedsolids.

Owing to the decanting primarily fine particles or small agglomeratesarrive in the overflow while primarily larger particles or agglomeratesare to be found in the underflow. For this reason the direct filtrationof the overflow is difficult on backwashing pressure filters or issometimes even impossible. The fine particles quickly tend to form athin and impermeable layer on the filter medium. An immediate increasein pressure and low filtration rates result. In addition it is difficultto remove the particle layer and this leads to a sharply limitedbackwashing capacity (capacity to regenerate) of the filter medium.

A method for purifying a backwashing pressure filter is known from U.S.Pat. No. 4,443,346 of the type which is suitable, for example, for usein the method according to the invention.

U.S. Pat. No. 3,497,452 describes a method using auxiliary filteringmeans, some of the filtrate being reprocessed for further filtration, inthe removal of solids from solvents in the chemical industry.

Although the formation and removal of a filter cake is described inknown methods, it is not described how or in what form the auxiliaryfiltering means is used. As is known, substances extraneous to theprocess of mineral (for example perlite, diatomite) or organic (forexample cellulose) origin are added as auxiliary filtering means insuspended form to the prefilt prior to the filter. The drawback is thatin known applications new auxiliary filtering means have to be added andaccumulate as an additional residue during disposal.

It is the object of the invention to provide an improved method forfiltering brines in chlor-alkali chemistry, a stream of materialaccumulating during the process itself being used as the filtering aid.

It is the object of the invention to provide an improved fully-automaticmethod for filtering brine that is suitable for chlor-alkalielectrolysis.

This object is achieved according to the invention in that, in a firststage, the raw brine is separated into a partial flux with a low solidscontent as clear brine and a partial flux with a high solids content viaa decanter, and in a second stage, some of the partial flux of the clearbrine, which has a high solids content, is added to the pressure filteras an auxiliary filtering means.

During filtration, the coarser particles of the partial flux with a highsolids content forms a porous and highly permeable filter cake.Problematical fine particles are intercalated and reliably retained. Thecoarse grain of the partial flux with a high solid content is thusassigned the function of an auxiliary filtering means. This results in aslow pressure increase and high filtration rates. The particle layerreaches a thickness of about 1 to 2 mm and can be easily removed andthis leads to a good backwashing capacity (capacity to regenerate) ofthe filter medium. High solids retention is ensured by the use ofsuitable, fine-pore filter media.

The dosage of the partial flux with a high solids content is asubstantially freely selectable parameter and increases the flexibilityof the filtration with respect to variations in the solids composition.

Because the clear brine to be filtered has a partial flux of thedecanter underflow added prior to entry into the pressure filter, thereis a displacement of the particle size distribution into the coarserrange. The displacement of the particle size distribution in cooperationwith backwashing pressure filters offers substantial technical andeconomical advantages:

-   -   Increase in the filtration output, i.e. saving in filter area    -   Improved separation of the solids from the filter medium    -   Reduced risk of clogging of the filter medium    -   Insensitivity to varying solids composition    -   Increased flexibility and reliability of the filtration stage.

The method is to be described in more detail with the aid of adiagrammatic view as the single FIG. 1.

In FIG. 1 a container with raw brine A to be purified is designated bythe reference numeral 1. The container 1 is connected via a linecontaining a clear brine B as overflow and connected to a secondcontainer 2. Leading from a lower part of the container 1 is the linecontaining slurry C with a high solids content leading partially to adisposal site, not shown. The other part of the slurry with a highsolids content leads to a metering pump 3. The lower part of thecontainer 2 is connected to a pump 4 via a line. The pressure line ofthe pump 4 contains a mixed brine D and leads to a backwashing pressurefilter 5 with a line E for the pure brine and a line F for theelutriation.

During operation the raw brine A is initially subjected in the container1 to a preliminary clarification, thickeners, so-called staticdecanters, generally being used. Owing to the large buffer. volume(frequently several thousand cubic metres) and the long dwell timeresulting therefrom, these decanters contribute to the operating safetyof electrolysis plant. To improve the sedimentation properties of thesolids a flocculation aid (for example polyelectrolyte) is generallyadded to the raw brine.

The raw brine A is separated in the container 1 into a raw brine B witha low solids content and an underflow C with a high solids content. Theclear brine B typically also contains solids in the range of 30 to 100ppm. Some of the underflow C with a high solids content is added to theclear brine B and thus generates a mixed brine D which is fed to thefilter 5 via the pump 4. The addition of the underflow may take place atvarious points, either on the suction or pressure side of the pump 4 oralso into the container 2 if a mixing device is available.

The pressure filter 5 generates a virtually solid-free filtrate E, therequired discharge values generally being fallen below without furtherpost-treatment (0.5 to 1 ppm). The retained solids are intermittentlydischarged as thickened slurry F and fed together with the underflow ofthe thickener, for example to a downstream dehydration process.

The invention will now be described in more detail with the aid of anembodiment and a comparison example. In both cases treatment of 100 m³/hraw brine with a solids content of 1,000 ppm is assumed.

Embodiment 1 Brine flux Raw brine Clear brine Thick slurry Mixed brinePure brine Elutriation Designation A B C D E F Concentration (ppm) 100050 150000 250 0.5 150000 Ratio of the solids concentration (flux D/B) 5Specific filter output pressure (l/m2 × h) 1000 filter 5 Required filterarea pressure (m2) 100 filter 5Result:

After the start of operation with feeding, a permanent increase in thefiltration output by the factor 5 to 6 could be found. The decanterunderflow was added in the process in different ratios. Stable andreliable operations could be observed at ratios of 5 to 10:1, based onthe solids loading, i.e. the solids concentration in the inlet D to thefilter 5 was 5 to 10 times higher than that of the clear brine B. Thefiltrate quality was clearly below 1 ppm, a value of 0.5 ppm was mainlyachieved.

COMPARISON EXAMPLE

(Without Feeding of Decanter Underflow) Brine flux Raw brine Clear brineThick slurry Mixed brine Pure brine Elutriation Designation A B C — E FConcentration (ppm) 1000 50 150000 — 0.5 150000 Ratio of the solidsconcentration (flux D/B) — Specific filter output pressure (l/m2 × h)200 filter 5 Required filter area pressure (m2) 500 filter 5Result:

It has been found that owing to the low output, filtration is impossiblefrom the point of view of economy. A clear tendency to clog was alsoestablished, which was shown by continuously decreasing filtrationoutput.

1. Method for continuously filtering raw brine (A) for use inchlor-alkali electrolysis, by means of a backwashing pressure filter(5), characterised in that in a first stage, the raw brine (A) isseparated into a partial flux with a low solids content as clear brine(B) and a partial flux (C) with a high solids content via a decanter(1), and in a second stage, some of the partial flux (C) of the clearbrine (B), which has a high solids content, is added to the pressurefilter (5) as an auxiliary filtering means.
 2. Method according to claim1, characterised in that the dosage of the partial flux (C) with a highsolids content is added directly to the clear brine (B) in a controlledmanner.
 3. Method according to claim 1, characterised in that abackwashing pressure filter (5) is used as a pressure filter.